Rollup merge of #123043 - GoldsteinE:fix/repr-c-dead-branches, r=oli-obk

Disable dead variant removal for `#[repr(C)]` enums.

This prevents removing dead branches from a `#[repr(C)]` enum (they now get discriminants allocated as if they were inhabited).

Implementation notes: ABI of something like

```rust
#[repr(C)]
enum Foo {
    Foo(!),
}
```

is still `Uninhabited`, but its layout is now computed as if all the branches were inhabited.
This seemed to me like a proper way to do it, especially given that ABI sanity check explicitly asserts that type-level uninhabitedness implies ABI uninhabitedness.

This probably needs some sort of FCP (given that it changes `#[repr(C)]` layout, which is a stable guarantee), but I’m not sure how to call for one or which team is the most relevant.

See https://github.com/rust-lang/unsafe-code-guidelines/issues/500.

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